In data transmission between central control units, such as microcontrollers used as a master device and peripheral components, such as sensors used as slave devices, serial bus systems are frequently used. In motor vehicles, for instance, the SPI (serial peripheral interface) bus is often used. A serial clock signal is supplied by the microcontroller to the connected sensors, in this context. A data line is provided for the data transfer from the microcontroller to the sensors (MOSI=master out slave in), a data line for the data transfer from the sensors to the microcontroller (MISO=master in slave out) and a selection line for the selection of one or more connected sensors (CSB=chip select). Various data protocols are known for this bus system that is based on four lines.
It was provided, for instance, that additional selection lines or chip select lines be provided, corresponding to the number of peripheral devices, which requires a plurality of select ports on the respective microcontroller. This makes possible that, in a select cycle, which is defined by a specified level of the select signal, both the request data rates and the corresponding response data frames are able to be transmitted via the MOSI and MISO lines, if a request and a response to it take place within one select cycle. However, the first, that is, highest-order bits of the response are not able to be allocated specifically to the request, since the selected slave device or peripheral device first has to, at least partially, evaluate the request data frame. Consequently, not all bits of the data frames are used efficiently.
A similar problem arises if only a single selection line is provided, and the selection of the selected peripheral component by address bits in the request data frame, that is sent via the MOSI line, is specified. If both the request data frame and the corresponding response data frame are sent in the same selection cycle, the first bits of the respective response data frame cannot be allocated in a request-specific manner. It was provided in addition, that one should send the request data frames and the response data frames in different selection cycles. With that, an equalization in time takes place between a request by the microcontroller and the corresponding response by the selected slave device or peripheral device. It is true that the complete data frame can then be used for the transmission of request-specific bits, but a halving of the data throughput comes about if each sensor is connected to a chip-select line, as is the case, for example, in the so-called Autoliv protocol.